1 look at memberlist of the clan, if 70% of the members is not online, look for a clan

2 look at memberlist, if less then 50% is not active, consider to start a new clan with your active players

3 if almost everybody is active but you CL is not for a long time, start a clanvote about who has to be the new CL. Make screenshots / prove how the clanvote was done and mail that to support with the question to replace CL according to your clanvote

Or the devs could program the game to automatically replace the inactive clan leader with either the oldest active member or the next-highest ranking member after the clan leader has been inactive for more than 30 or 45 or 60 days (pick one).